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Evaluation of Pseudopregnancy in the American Black Bear (Ursus Americanus)
S. Clark-Deener, J.B. Mesa, T...
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The American black bear (Ursus americanus) exhibits several peculiar reproductive strategies that show high synchronicity with the environment. For instance, mating season occurs from June through August, months with the highest food productivity. Soon after, the fertilized embryos (blastocysts) arrest their development until mid-to-late November when implantation occurs. Thereafter, active gestation lasts 60 days. Delay in embryo development allows black bears to carry their pregnancy and give birth while hibernating over winter. Moreover, it is believed that black bears can experience pseudopregnancy. It is currently assumed that the primary corpus luteum is the main source of progesterone (P4) during active pregnancy until parturition. Previous studies conducted on various bear species, including black bears, have shown similar patterns in serum P4 profiles in females that gave birth and those that did not give birth. However, such studies in black bears did not include any pregnancy diagnoses, which could have resulted in false negative observations. Pregnant females that experience fetal death could display similar P4 profiles to those that produced live offspring. The aim of the study was to use serum P4 profiles to determine whether female bears that were never detected as pregnant by ultrasonography experience pseudopregnancy (defined as the maintenance of luteal P4 production, in the absence of pregnancy, for periods and concentrations similar those of pregnant females). Serum samples were collected from 8 female black bears (5 non-pregnant and 3 pregnant that did not give birth to cubs; mean ages 10.8 and 8yrs, respectively). Bears were captured by the Virginia Department of Game and Inland Fisheries and housed at Virginia Tech’s Black Bear Research Center between October and February for various years from 1989 to 2005. During this time, bears were anesthetized every 10 days for ultrasonography and collection of blood samples. Blood samples were centrifuged and stored at - 20°C until measurement of serum P4 by the 125I Coat-A-Count progesterone immunoassay. Contrary to previous studies, P4 concentrations of pregnant and non-pregnant females were significantly different (P<0.001). During the pre-implantation period there was a 2 and 2.5 fold increase in P4 in non-pregnant and pregnant bears, respectively. During periods of active pregnancy concentrations of P4 were significantly different, with pregnant bears showing a 5-fold increase in P4 after embryonic implantation, while non-pregnant bears showed a gradual decrease in P4, returning to baseline within 20 days of the pre-implantation peaks. These results suggest that black bears are unlikely to exhibit pseudopregnancy.
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Affiliation of the authors at the time of publication
Department of Large Animal Clinical Sciences, Virginia-Maryland Regional College of Veterinary Medicine and
Department of Fish and Wildlife Conservation, Virginia Tech, Blacksburg, VA
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